In charged particle systems, comprising both electron microscopes and focused ion beam systems, a column is typically used to focus a charged particle beam onto the surface of a target which is to be imaged and/or processed using the beam. The energy of the beam when it strikes the target surface influences the beam interaction with the substrate and also the characteristics of the image acquired. For example, some imaging applications require the highest spatial resolutions in order to spatially resolve the smallest features on the target, in some cases down to atomic resolution. In other imaging applications, it is necessary to use lower beam energies to reduce damage to the target or to image only the surface of the target (since higher energy beams typically penetrate deeply into the target).
Similar considerations apply for beam-enhanced processing of targets, such as ion milling, beam-induced deposition or etching (using either electrons or ions), ion polishing, ion implantation, etc. For all these processing methods, the beam energy when it encounters the target surface may have substantial effects on the characteristics of the process, including the spatial resolution (i.e., the size of the milled or deposited features), as well as the processing rate, surface smoothness, density of depositions, depth of damaged layers, etc.
A common characteristic of charged particle columns is the beam voltage at which the optical design was optimized, where “optimization” may correspond to the focusing ability of the column to produce the smallest possible beam diameter for a pre-determined beam current, or the highest beam current within a pre-determined beam diameter. When a range of beam energies at the target is desired (e.g., when both imaging and processing are to be performed on the same target), the best performance for imaging or processing can typically only be obtained for beam energies near this optimized beam energy. For all other beam energies, the imaging or processing performance is lower than would have been possible had the column been optimized for the particular beam energy. “Lower performance” in this context would mean a larger beam diameter for a pre-determined beam current, or a lower beam current within a pre-determined beam diameter, while “improved performance” will be used to mean a smaller beam diameter for a pre-determined beam current, or a higher beam current within a pre-determined beam diameter.
What is needed is a charged particle column having improved performance at multiple beam energies. Another desirable goal would be to configure a charged particle column for improved performance over a wider range of energies, spanning those energies which have been determined to be optimal for imaging and also beam energies which have been determined to be optimal for beam processing applications.